Turf grass (sod) is a living organism that must be handled properly to ensure its survival when it is removed from one location and transplanted to another. Sod is generally harvested using large machinery that cuts slabs of sod from the soil. Typically, a sod harvester employs one or more inclined conveyors to elevate the sod from the ground to a horizontal conveyor. A stacking head is then used to pick up the sod from the horizontal conveyor and stack it on a pallet.
Two general types of harvesters exist: slab harvesters; and roll harvesters. A roll harvester forms the slabs of sod into rolls which are then accumulated on the horizontal conveyor for pick up. A slab harvester, on the other hand, directly stacks the slabs in their flat form. With either type of harvester, the speed at which the harvester can remove sod from the horizontal conveyor and stack it oftentimes is the controlling factor that determines how fast the harvester can operate.
FIGS. 1A-1C illustrate an example of a portion of a prior art roll harvester 100. Roll harvester 100 includes a cutting head comprised of a first blade 111, a second blade 112, and a roller 113. Blade 111 is periodically lowered into the sod to make vertical cuts defining the length of the slabs. Blade 112 oscillates back and forth underneath the sod to sever the slab from the underlying soil 101. Roller 113 applies pressure to the sod as it is cut to facilitate the cutting of clean slabs.
Slabs cut by the cutting head are routed to conveyor 110a which lifts the slabs up towards conveyor 110b. A rolling mechanism 130 is positioned above conveyor 110a and causes the front edge of slabs 102 to be rolled backwards as the slabs advance towards conveyor 110b. The completed rolls 102 are then advanced onto conveyor 110b where they are accumulated until a sufficient number of rolls are present on the conveyor.
FIG. 1A illustrates the instance where four rolls have been accumulated on conveyor 110b. As best shown in FIG. 1B, a stacking head 120 is positioned above conveyor 110b and is designed to pick up four rolls of sod when five rolls of sod are present on conveyor 110b. The use of four and five rolls in this example is entirely arbitrary. This example would be equally applicable if stacking head 120 were configured to pick up any other number of rolls when any other number of rolls was present on conveyor 110b. For example, stacking head 120 could be positioned to the right of what is shown in FIGS. 1A-1C to allow it to pick up four rolls when four rolls were present on conveyor 110b. Similarly, if a longer conveyor were used, stacking head 120 could be configured to pick up four rolls when seven rolls were present on conveyor 110b, or stacking head 120 could be wider to pick up five rolls when six rolls were present on conveyor 110b. 
As shown in FIG. 1B, once the proper number of rolls (e.g. four) is positioned underneath stacking head 120, stacking head 120 descends to pick up the rolls. Stacking head 120 can include various mechanisms for picking up the rolls, but the specific mechanism is not important to the present invention. Of importance is the amount of time required to pick up the rolls. Specifically, once conveyor 110b is advanced so that four rolls are positioned underneath stacking head 120, conveyor 110b cannot be advanced again until stacking head 120 has lifted the four rolls from conveyor 110b. For example, if conveyor 110b were advanced prior to stacking head 120 securing the rolls (e.g. by clamping or piercing the rolls), the leftmost roll would fall off conveyor 110b. Also, if conveyor 110b were advanced while stacking head 120 has secured the rolls but prior to lifting them, the rolls could be ripped or otherwise damaged as conveyor 110b pulls the rolls away from stacking head 120.
At the same time, if a subsequent roll (e.g. one on conveyor 110a) has been completed and needs to be advanced onto conveyor 110b, but stacking head 120 has not yet picked up the rolls, the harvester will be forced to temporarily stop to wait until conveyor 110b can be advanced to make room for the subsequent roll. In other words, if a newly formed roll on conveyor 110a cannot be advanced to conveyor 110b, the harvester will need to stop (or at least slow down) so that a new slab does not advance into roll mechanism 130. Otherwise, if the harvester is not slowed or stopped, the rolls would jam together clogging conveyor 110b. 
FIG. 1C represents the case where stacking head 120 has descended to pick up the rolls, but has not yet secured them or lifted them from conveyor 110b. Also, FIG. 1C shows that a roll 102a is ready to be advanced onto conveyor 110b while a new slab 102b is advancing up conveyor 110a. FIG. 1C therefore represents one case when harvester 100 will have to stop to wait for stacking head 120 to pick up the rolls. Specifically, because slab 102b will continue to advance towards roll mechanism 130 as long as harvester 100 continues moving forward (or as long as inclined conveyor 110a continues to rotate), but roll mechanism is not ready to receive slab 102b because roll 102a cannot be advanced onto conveyor 110b, harvester 100 will have to temporarily stop until stacking head 120 has removed the rolls thereby allowing conveyor 110b to be advanced to receive roll 102a. 